Bielders, C., Delvaux, B., & De Backer, L. (1990). Particle Density of Volcanic Soils As Measured With a Gas Pycnometer. Soil Science Society of America Journal, 54(3), 822-826. https://doi.org/10.2136/sssaj1990.03615995005400030034x (Original work published 1990)